Spacer grid using tubular cells with mixing vanes

ABSTRACT

A spacer grid specifically designed for accident tolerant fuel utilizing fuel rods with SiC cladding for implementation in pressurized water reactors. The spacer grid has a generally square design that allows for ease of SiC fuel rod insertion during the fuel assembly fabrication process by providing a smooth contact geometry. The co-planar support allows the fuel rods to be rotated axially more freely at the grid location than a conventional six-point contact geometry used in existing fuel assembly designs.

BACKGROUND 1. Field

This invention pertains generally to nuclear fuel assemblies and, morespecifically, to nuclear fuel assemblies that employ fuel rods withceramic claddings.

2. Related Art

A typical nuclear power reactor includes a reactor vessel housing anuclear reactor core. Spaced radially, inwardly from the reactor vesselwithin the interior of the vessel is a generally cylindrical core barreland within the barrel is a former and a baffle system (hereafter calledthe “baffle structure”), which permits transition from the cylindricalbarrel to a squared-off periphery of the reactor core formed by the fuelassemblies arrayed therein.

The reactor core is composed of a large number of elongated fuelassemblies. Each fuel assembly includes a plurality of fuel rodscontaining the fissile material, which reacts to produce heat. The fuelrods of each fuel assembly are held in an organized, spaced array by aplurality of grids, spaced axially along the fuel assembly length andattached to a plurality of elongated control rod guide thimbles of thefuel assembly.

During operation of the reactor, a coolant fluid such as water istypically pumped into the reactor vessel through a plurality of inletnozzles. The coolant fluid passes downward through an annular regiondefined between the reactor vessel and the core barrel, turns in a lowerplenum defined in the reactor vessel, then passes upwardly through thefuel assemblies of the reactor core, and exits from the vessel through aplurality of outlet nozzles extending through the core barrel. Heatenergy, which the fuel rods of the fuel assemblies impart to the coolantfluid, is carried off by the fluid from the vessel. Due to the existenceof holes in the core barrel, coolant fluid is also present between thebarrel and a baffle structure and at a higher pressure than within thecore. However the baffle structure, together with the core barrel, doseparate the coolant fluid from the fuel assemblies as the fluid flowsdownwardly through the annular region between the reactor vessel andcore barrel.

Also, the power within the core is limited by the hottest temperaturethat can be endured by the fuel assembly components without failure.Traditionally, that limiting temperature is the temperature of the fuelrod zirconium cladding in the fuel rod experiencing the hottesttemperature within the core. To promote lateral mixing of the coolantwithin the core to achieve a more radially uniform temperature the fuelassemblies typically employ mixing vanes that also tend to inducelateral movement of the coolant that could also result in vibration ofthe fuel rods if unrestrained. Typically, springs and dimples areemployed in conventional grid structures within the fuel assemblies, toprovide a six point contact arrangement between the fuel rods and gridstructure in which the springs bias the fuel rods against a pair ofdimples, with the springs and dimples extending from opposing walls offuel rod support cells of the grids. The grids are usually axiallyspaced in tandem along the fuel assemblies to maintain lateral spacingbetween the fuel rods, through which the coolant flows. The lateral flowof coolant achieves a more uniform radial temperature distribution inthe core and allows for a higher power output than could be obtainedwithout the cross-flow of coolant, however, the power output is stilllimited by the highest temperature experienced by the fuel rods'zirconium cladding.

More recently, SiC cladding has been proposed for the fuel rods, becausethe SiC cladding can withstand a much higher temperature than zirconium.In one embodiment, the SiC is formed in a braded structure toaccommodate some flexibility and avoid the conventional rigidity ofceramics. Since the SiC fuel rods typically have very rough surfaces asshown by reference character 66 in FIG. 3, it is likely that the currentgrid designs illustrated in FIG. 1, which employ grid spring/dimplefeatures for holding the fuel rods within the fuel assembly, could bedamaged during insertion into the fuel skeleton. The potential fordamage is especially a cause for concern for the horizontal dimpledesign which is typically used in most pressurized water reactor grids.In the case of the horizontal dimple design, the rough surface of theSiC cladding could hang-up on the dimple and damage the dimple, or itcould damage the SiC fuel rod cladding. Therefore, the current six-point(two springs and four dimples) grid design may not be appropriate foruse with SiC fuel cladding. Accordingly, a new fuel assembly grid designis desired that is specifically suitable for SiC clad nuclear fuel.

SUMMARY

The invention contemplates a nuclear fuel assembly spacer grid having anaxial dimension along the vertical axis of an elongated fuel assembly inwhich it is to be deployed. The nuclear fuel assembly grid comprises aplurality of tubular fuel rod support cells having four walls, generallysquare in cross section. The walls have a length along the axialdimension that is longer than a width of the walls and substantiallyflat corners that connect on an interior of the spacer grid withadjacent fuel rod support cells or a control rod support cell, with aninterior of each of the walls supporting a vertical spring. Theinvention also contemplates a mixing vane connected to an exterior of atleast one of the fuel rod support cells in an area between fuel rodsupport cells.

In one embodiment each of the fuel rod support cell walls are bowedinwardly, preferably around an axis parallel to the vertical axis, intoan interior of the fuel rod support cell. Preferably, the verticalspring is formed from two vertical slits in the center of each of thewalls of the fuel rod support cells.

In still another embodiment the corners between connected adjacent fuelrod support cells are integral to both adjacent support cells.Preferably, the integral corners are not substantially thicker than0.018 inch (0.046 cm).

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the invention can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

FIG. 1 is a top plan view of a pressurized water reactor conventionalfuel assembly grid;

FIG. 2 is an elevational view, partially in section, of a conventionalfuel assembly; the assembly being illustrated in verticallyforeshortened form with parts broken away for clarity;

FIG. 3 is a perspective view of SiC fuel rod cladding;

FIG. 4 is a perspective schematic view of one fuel rod tubular supportcell of one embodiment of this invention;

FIG. 5 is a perspective schematic view of a plurality of the fuel rodtubular support cells shown in FIG. 4, attached at their corner withportions of the fuel rod cladding supported within the cells;

FIG. 6 is a perspective of one embodiment of a mixing vane configurationthat can be employed with this invention;

FIG. 7 is a side view of another embodiment of a mixing vane that can beemployed with this invention;

FIG. 8 is a planned view of the mixing vane of FIG. 7 installed in thetubular grid of FIG. 5; and

FIG. 9 is a schematic perspective view of the mixing vanes of FIG. 6installed over the tubular grid of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Many conventional spacer grids are composed of straight grid straps thatare interleaved together to form an egg-crate configuration having aplurality of roughly square cells, many of which support fuel rods. Anexample of such a conventional fuel grid 10 can be found in FIG. 1. Aspaced, parallel array of grid straps 12 of equal length are positionedorthogonally to a second plurality of the spaced, parallel grid straps14 of equal length and are encircled by a border strap 18, with each ofthe straps being welded at their intersections. The cells 16 support thefuel rods while the cells 20 support guide tubes and an instrumentationtube. Because the fuel rods must maintain a spacing or pitch betweeneach other, these straight grid straps 12 and 14 at the locations thatborder the cells 16 that support the fuel rods have springs 22 and/ordimples 24 that are stamped in the sides of the straps 12 and 14 toprotrude into the cells 16 to contact the fuel rods and hold them firmlyin position. The stamped features on the grid straps 12 and 14, i.e.,the springs 22 and the dimples 24, require careful design and precisemanufacturing to assure adequate force is maintained to secure the fuelrods when considered in combination with the other grids in the tandemarray of grids along the fuel assembly.

Referring to FIG. 2, there is shown an elevational view of aconventional fuel assembly represented in vertically foreshortened formand being generally designated by reference character 40. The fuelassembly 40 is of the type used in a pressurized water reactor andbasically includes a lower end structure or bottom nozzle 42 forsupporting the fuel assembly on a lower core plate (not shown) in thereactor core region and a number of longitudinally extending guidethimbles or tubes 44 which project upwardly from the bottom nozzle 42.The assembly 40 further includes a plurality of the transverse grids 10,shown in FIG. 1. This invention replaces the fuel rod support cells 16shown in FIG. 1 with the tubular fuel rod support cells 26 shown inFIGS. 4 and 5. The grids 10 are axially spaced along and supported bythe guide thimbles 44. The grids 10 supported in a spaced tandem arrayby the control rod guide thimbles 20 and coupled at their lower ends tothe bottom nozzle 42 is generally referred to as the fuel assemblyskeleton. Assembly 40 also includes a plurality of elongated fuel rods36 transversely spaced and supported in an organized array by the grids10. Also, the assembly 40 has an instrumentation tube 46 located in thecenter thereof and an upper end structure or nozzle 48 attached to theupper ends of the guide thimbles 44. With such an arrangement of parts,the fuel assembly 40 forms an integral unit capable of beingconveniently handled without damaging the assembly of parts.

As mentioned above, the fuel rods 36 and the array thereof in theassembly 40 are held in spaced relationship with one another by thegrids 10 spaced along the fuel assembly length. Each fuel rod 36includes nuclear fuel pellets 50 and the opposite ends of the rods 36are enclosed by upper and lower end plugs 52 and 54, to hermeticallyseal the rod. Commonly, a plenum spring 56 is disposed between the upperend plug 52 and the pellets 50 to maintain the pellets in a tight,stacked relationship within the rod 36. The fuel pellets 50 composed offissile material are responsible for creating the reactive power of thePWR. A liquid moderator/coolant, such as water or water-containingboron, is pumped upwardly through the fuel assemblies of the core inorder to extract heat generated therein for the production of usefulwork.

To control the fission process, a number of control rods 58 arereciprocally movable in the guide thimbles 44 located at predeterminedpositions in the fuel assembly 40. Specifically, the top nozzle 48 hasassociated therewith a rod cluster control mechanism 60, having aninternally threaded cylindrical member 62 with a plurality of radiallyextending flukes or arms 64 such that the control mechanism 60 isoperable to move the control rods 58 vertically in the guide thimbles 44to thereby control the fission process in the fuel assembly 40, all in awell-known manner.

As shown in FIGS. 4 and 5, one preferred embodiment of the spacer grid10 of this invention comprises a plurality of tubular cells 26 withmixing vanes 28 (shown in FIGS. 7, 8 and 9) with the cells attachedtogether at their corners 30 with the corners preferably formed fromflat vertical strip of the strap that surrounds the fuel rod supportcells 16. The tubular cell 26 shown in FIG. 3 has four vertical springs32, one in the center of each cell wall 34 to prevent spring damagesduring fuel rod insertion. Also, the co-planar springs 32 will notprevent fuel rod local rotation such that it could otherwise cause fuelrod breakage due to fuel rod bowing. In one embodiment, the surface ofthe vertical springs is coated with a hard material at least as hard orharder than the SiC, cladding, such as the Diamond-Like Coatingavailable from Techmetals, Dayton Ohio, or other similar material, toprevent the rough SiC cladding from wearing down the spring. The coatingmay be applied by physical vapor deposition or other similar coatingprocesses. Preferably, each wall 34 of the fuel rod support cells 16 isbowed inwardly towards the corresponding fuel rod 36, with the bendcentered on a vertical axis running along the vertical spring 32. Thetubular fuel rod support cells 26 take the place of the conventional sixpoint contact support cells and in other respects the tubular supportgrid, e.g., the control rod support locations, can take the form of atraditional grid or replicate the tubular fuel rod support cells, withor without the vertical springs.

As an example, a typical split-mixing vane design 28 can be attachedusing the proposed geometry in FIG. 4. The vane will be located at thesub-channel 68 between fuel rods, i.e., the open area between the fuelrod tubular support cells 26, as shown in FIGS. 7 and 8. A chamfer maybe formed under the vane to reduce pressure drop of the grid. The gridmixing vane will be integrated with the tubular cell as a single body.In addition, advanced mixing vane designs such as multiple angled vane,non-flat vane (convex or concave), or swirling vane (FIG. 7) can beadditively manufactured as a single body structure. By introducing theadditive manufacturing (3D printing), the proposed spacer grid can beprinted without further assembling or welding processes. In addition,there will be only single thin wall (0.018 inch (0.046 cm)) betweencells so that the pressure loss coefficient would be smaller than thatof a typical grid design.

Accordingly, this invention provides a tubular cell design which allowsfor a smooth insertion of SiC type fuel rods while also resulting in alow pressure drop as compared to existing grid designs. Such anadditively manufactured spacer grid design allows for 1) theimplementation of highly detailed yet fully integrated mixing featuresenhancing thermal and hydraulic performance, 2) minimizing the overallpressure drop (single wall) and 3) increasing overall grid strength forseismic concerns.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular embodiments disclosed are meant to be illustrative only andnot limiting as to the scope of the invention which is to be given thefull breadth of the appended claims and any and all equivalents thereof.

What is claimed is:
 1. A nuclear fuel assembly spacer grid having anaxial dimension along a vertical axis of an elongated nuclear fuelassembly in which it is to be deployed, the nuclear fuel assembly spacergrid comprising: a plurality of fuel rod support cells having fourwalls, generally square in cross section, having a length along theaxial dimension that is longer than a width of the walls and cornersthat connect with adjacent fuel rod support cells or a control rodsupport cell, with an interior of each of the walls of the fuel rodsupport cells supporting a vertical spring; and a mixing vane connectedto an exterior of at least one of the fuel rod support cells, whereineach of the fuel rod support cell walls are bowed inwardly, along anentire length thereof, into an interior of the fuel rod support cell. 2.The nuclear fuel assembly spacer grid of claim 1 wherein the fuel rodsupport cell walls are bowed symmetrically around an axis parallel tothe vertical axis.
 3. The nuclear fuel assembly spacer grid of claim 2wherein at least one of the vertical springs is formed from two verticalslits in a center portion of at least one of the walls of at least oneof the fuel rod support cells.
 4. The nuclear fuel assembly spacer gridof claim 1 wherein each vertical spring is formed from two verticalslits in a center portion of each of the walls of the fuel rod supportcells.
 5. The nuclear fuel assembly spacer grid of claim 1 wherein atleast one of the vertical springs is coated with a material.
 6. Thenuclear fuel assembly spacer grid of claim 5 wherein the material is acarbon coating.
 7. An elongated nuclear fuel assembly, comprising: aspacer grid having an axial dimension along a vertical axis of theelongated nuclear fuel assembly, the spacer grid comprising: a pluralityof fuel rod support cells having four walls, generally square in crosssection, having a length along the axial dimension that is longer than awidth of the walls and corners that connect with adjacent fuel rodsupport cells or a control rod support cell, with an interior of each ofthe walls of the fuel rod support cells supporting a vertical spring;and a mixing vane connected to an exterior of at least one of the fuelrod support cells, wherein each of the fuel rod support cell walls arebowed inwardly, along an entire length thereof, into an interior of thefuel rod support cell; and a fuel rod positioned in contact with atleast one of the vertical springs.
 8. The elongated nuclear fuelassembly of claim 7 wherein the fuel rod support cell walls are bowedsymmetrically around an axis parallel to the vertical axis.
 9. Theelongated nuclear fuel assembly of claim 7 wherein each vertical springis formed from two vertical slits in a center portion of each of thewalls of the fuel rod support cells.
 10. The elongated nuclear fuelassembly of claim 7 wherein the at least one of the vertical springs iscoated with a material having a hardness that is equal to or greaterthan the hardness of a SiC cladding on the fuel rod.
 11. The nuclearfuel assembly of claim 10 wherein the material is a carbon coating. 12.The nuclear fuel assembly of claim 8 wherein the at least one of thevertical springs is formed from two vertical slits in a center portionof at least one of the walls of at least one of the fuel rod supportcells.